mersenneforum.org Prime95 and Dual Processors
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 2006-06-13, 15:22 #1 AntonVrba     Jun 2005 2·72 Posts Prime95 and Dual Processors I now run a 2 x Dual XEON 3,6GHZ system and have noted that Prime95 does not take advantage of the two processors 1) Best time for 4096K FFT length: 125.782 ms. while running PFGW in parallel and 2) Best time for 4096K FFT length: 124.827 ms. when PFGW is stopped. In case 1) the windows task manager shows 50% CPU utilisation and in case 2) 25% CPU utilisation. Similar to Prime95, PFGW did not show any slow down either. regards - Anton
 2006-06-14, 00:23 #2 dsouza123     Sep 2002 10100101102 Posts Possible scenario: Hyperthreading on, Windows sees 4 CPUs, using 100 % of 1/2 of total CPU utilization is 50%, using 100 % of 1/4 of total CPU utilization is 25%. Really using 100% of a single physical HT CPU when Prime95 is running. The hyperthreading only gives a small percent of true extra CPU capacity if one application is using FP and another integer. For one core, if only one program is using FP then it uses 100% of the CPU and the second virtual HT 0%.
2006-06-14, 04:40   #3
AntonVrba

Jun 2005

2·72 Posts

Quote:
 Originally Posted by dsouza123 Possible scenario: For one core, if only one program is using FP then it uses 100% of the CPU and the second virtual HT 0%.
Your explanation is correct and logical.

FFT math is very suitable for parallel processing, lots of parallel small multiplications that are combined at the end.

So the point is that Prime95 takes no advantage of the fact that parallel processors do exist.

regards

2006-06-14, 10:10   #4
rgiltrap

Apr 2006
Down Under

89 Posts

Quote:
 Originally Posted by AntonVrba So the point is that Prime95 takes no advantage of the fact that parallel processors do exist.
Or you could simply have two instances of Prime95 running to ensure 100% utlization, which is probably a great deal less effort than trying to parallelize the code

2006-06-14, 14:05   #5
fetofs

Aug 2005
Brazil

2·181 Posts

Quote:
 Originally Posted by AntonVrba Your explanation is correct and logical. FFT math is very suitable for parallel processing, lots of parallel small multiplications that are combined at the end. So the point is that Prime95 takes no advantage of the fact that parallel processors do exist. regards
Quote:
 Originally Posted by George Woltman I own a dual-CPU computer or several computers networked together. Can these CPUs work together on the same exponent to get results faster? No. It is not uncommon for the program to read and write nearly 100MB of data every second. This is far too much data to transfer around the network in a timely manner. Although, a program could be written for dual-CPU systems (it would be quite time-consuming), the machine will still get more throughput by working on separate exponents.
.

 2006-06-14, 18:29 #6 AntonVrba     Jun 2005 2·72 Posts Hyperthreading - yes or no I now disabled hyperthreading on my 2 x dual xeon 3.6 GHz system Best time for 4096K FFT length: 121.698 ms 3.2 mS faster than if hyperthreading is enabbled. Conclusion - hyperthreading adds 3% overhead on the so called dual core processors - food for thought
 2006-06-14, 19:49 #7 kdelisle2005   Mar 2006 Michigan 11 Posts Run PRIME95 with Affinity With your dual core, have you tried the Affinity parameter with multiple executions of Prime95?? I have a P4 HT (1 cpu) and I run two executions of Prime95 (I renamed to prime95a.exe and prime95b.exe, in separate subdirectories) - using the Affinity parameter, CPU 0 and CPU 1, respectively. I know the one CPU can only work on one thing at a time but it seems I get better total throughput (be it very small) when trending for a couple of days. The downside to running two executions of Prime95 is slower response from other applications (Excel, IE, etc). Windows XP does a good job of handling the priority but it does take some time especially with two Primes going. For kicks, it sounds like you could run four executions of PRIME95. kevin

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